This application corresponds to Japanese Patent Application No. 10-147351, filed on May 28, 1998, which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a chip type electronic part to be mounted on a circuit substrate, more specifically to a chip type electronic part characterized in a solder to be used in an external electrode of the electronic part.
2. Description of the Related Art
A mounting step for soldering a chip type electronic part onto a circuit substrate is conducted as follows. That is, so-called reflow soldering is conducted by placing a chip type electronic part having an external electrode comprising a solder at the outermost layer on a cream solder applied on a circuit pattern of a circuit substrate, heating the chip type electronic part and the circuit substrate in a reflow furnace in this state so as to melt the solder of the external electrode and the cream solder of the circuit substrate, and cooling off for jointing the chip type electronic part with the circuit substrate.
However, in the above-mentioned jointing method, a chip standing failure called tombstone can be generated. The causes for generating such a chip standing failure include the followings.
(1) Change in the set temperature of the reflow furnace.
(2) Unevenness in the solderability caused by dirt of the external electrode of the chip type electronic part.
(3) Difference of the melting rates of the solders used in the external electrode and the circuit substrate of the chip type electronic part according to the composition difference of each solder.
An object of the present invention is to provide a chip type electronic part without the risk of generating a tombstone at the time of being mounted onto a circuit substrate.
A chip type electronic part according to the present invention comprises an electronic part element provided with external electrodes at both end parts thereof. The external electrodes comprise an electrode at the base layer formed as a thin film, and a solder at the outermost layer. The solder at the outermost layer contains Sn and Pb as the main component and 0.1 to 0.4% by weight of Ag.
In the chip type electronic part according to the present invention, the solid phase point of the above-mentioned solder at the outermost layer is lower by up to xe2x88x922xc2x0 C. than, the same as, or higher than the solid phase point of a cream solder to be used in a circuit substrate for mounting.
Accordingly, a chip type electronic part without the risk of generating a tombstone at the time of mounting the chip type electronic part on a circuit substrate can be obtained.